The long-term objective of our on-going and proposed research has been to stand mechanisms which govern the seasonality and persistence of mosquito-borne arboviruses at temperate latitudes. Temperature is believed to be the principal factor that impacts the transmission of arboviruses as it regulates the mosquito vector attributes of seasonal abundance, frequency of vertebrate host contact, duration of virus extrinsic incubation, susceptibility to virus infection and modulation of acquired virus infection. Thus, our proposed new field and laboratory investigations specifically will assess the impact of temperature on the ecology and genetics of Culex tarsalis Coquillett and its ability to transmit western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses in the irrigated desert valleys of California. The principal hypothesis addressed by the proposed research is that the 5 degrees C warmer mean annual temperature in the southeastern valleys of California has altered the genetics, ecology and vector competence of Cx. tarsalis sufficiently to allow persistent annual WEE and SLE virus activity, compared to the more northern and cooler San Joaquin Valley where the activity of these .viruses is now quite sporadic. This hypothesis will be tested by initiating new research with 4 specific aims: 1) determine if vector populations in the Coachella and San Joaquin Valleys differ in their tolerance and response to temperature during immature and adult stages; 2) elucidate physiological and behavioral mechanisms which allow vector populations in southeastern valleys to survive during both summer and winter conditions; 3) assess the impact of temperature on the vector's susceptibility to infection, duration of extrinsic incubation and modulation of WEE and SLE virus infections in field populations from the Coachella Valley; and 4) detect seasonal and spatial differences in the genetic structure of vector populations in the Coachella and San Joaquin Valleys. We believe our proposed studies address a major need for carefully designed field and laboratory experimentation to evaluate the importance of temperature as a major factor that controls vector and pathogen interrelationships.